Molecular Genetics

Meiotic recombination is necessary for proper segregation of chromosomes at meiosis I.

Facts

Homologous recombination occurs in all organisms. In eucaryotes, homolgous recombination occurs at a much higher frequency during meiosis than during mitosis. In meiosis I, the two halves of the bivalent must separate to opposite poles, so that each daughter cell receives one of the bivalent halves.

Ustilago maydis Recombination frequencies meiotic 1.9 x 10-3 mitotic 2.9 x 10-7

Meiotic bivalents line up stably between spindle poles with centromeres attached to opposite poles.	Maloriented bivalents are pulled towards one pole, dissociate from the spindle and then try again.	Maloriented bivalents held in position with a micromanipulator needle remain stably associated with the spindle (Nicklas & Koch (1969) JCB 43: 40-50).

• In most organisms there are one to three chiasmata per chromosome regardless of chromosome length or length of homology (limited to pseudoautosomal regions in sex chromosomes). On the other hand, a few organisms produce many chiasmata per chromosome (on average, 12), the actual number varying widely from chromosome to chromosome and meiosis to meiosis. These organisms include Aspergillus nidulans and Schizosaccharomyces pombe. Mature synaptonemal complexes are not formed during meiosis in these organisms.

Interpretations

• Recombination frequency
  • Special pairing mechanisms participating only in meiosis contribute to the difference in recombination frequency between meiosis and mitosis.
  • Other factors may also contribute. For example, spo11 mutants of S. cerevisiae have defects in meiotic recombination, but not in mitotic recombination or chromosome pairing.
  • Many argue that meiotic recombination must serve a purpose. Some point out that recombination speeds evolution. Yet, recombination is not inherently required for evolution.
• Bivalent separation
  • Separation of bivalents is unique to meiosis. Some aspect of this process may account for the higher frequency of recombination than in mitosis.
  • Chiasmata are necessary for stability of bivalent association with the spindle. This stability is necessary for proper segregation at meiosis I.
• Chiasmata per chromosome
  • Chiasmata are necessary for proper chromosome disjunction in meiosis I.
  • There are at least two ways that the presence of at least one chiasma on each chromosome pair can be achieved (ref).
    • In organisms without synaptonemal complexes, the average number of chiasmata per chromosome is so large that chromosomes without a chiasma occur only rarely.
    • The number of chiasmata could be regulated. A synaptonemal complex may inhibit development of multiple chiasmata, thus ensuring the presence of a single chiasma. Genetically, the inhibition is reflected as cross-over interference.

Further information

• Current concepts of recombination are a synthesis of conclusions from morphological, genetic and molecular investigations.
• Two proteins, Ipl1 and Aurora-B kinase, known to be required for proper segregation, appear to catalyze release of kinetochores from microtubules if they are not under tension (rev).